Joint event location and 3D Poisson's ratio tomography for downhole microseismic monitoring

Abstract

Passive seismic tomography plays a significant role in monitoring subsurface structures and properties during hydraulic fracturing. In this study, we develop a new passive seismic tomography approach to jointly invert for event locations, 3D P-wave velocity (V_p) and Poisson's ratio models, for downhole microseismic monitoring. The method enables to directly obtain the 3D Poisson's ratio or V_p/V_s ratio without the assumption of identical P- and S-wave raypaths. The back azimuths of passive seismic events are incorporated into the proposed method to better constrain the event locations. The 3D cross gradients are further applied to the proposed method to assimilate the P-wave velocity model with Poisson's ratio model in the same geological structure. The synthetic experiment demonstrates that the proposed tomographic method can recover the event locations and their adjacent 3D P-wave velocity as well as Poisson's ratio models effectively. In the field experiment, microseismic events are relocated reasonably well compared with the grid search solutions in a calibrated layer model. The area with low Poisson's ratios may be utilized to estimate the stimulated reservoir volume and indicate a potential area associated with highly saturated hydrocarbon.